When a closely-coiled helical spring of mean diameter (D) is subjected to an axial load (W), the stiffness of the spring is given by

• A. <img src="/_files/images/mechanical-engineering/strength-of-materials/113-316-1.png">
• B. <img src="/_files/images/mechanical-engineering/strength-of-materials/113-316-2.png">
• C. <img src="/_files/images/mechanical-engineering/strength-of-materials/113-316-3.png">
• D. <img src="/_files/images/mechanical-engineering/strength-of-materials/113-316-4.png">